Once I had them side by side and took a close look, I was pretty sure that Blue Lagoon was not the Riva alternative I was searching for, though Sky showed promise. Here are bottle closeups (Revlon first, then Chanel, then Hard Candy):
For comparison purposes, I used three coats of each polish, no top coat. I was going to crop my index finger out of the shot below since it's not needed to show the differences between the colors but then I thought "no, I'm going to leave it in so people can see what a mess Riva can be".
Yeah, so Blue Lagoon is no match for Riva. It's brighter and more opaque, with larger and more irregular shimmer. Sky is also more opaque than Riva but much closer to it in color, just bit lighter perhaps. Adding some clear to Sky might make it resemble Riva more, but that would probably make it streakier, and if I want streaks, I can just use Riva.
This won't be the last Riva comparison I do; I have Barry M Blue Moon in my possession but need to wait to try it until my swap partner gets the things I sent her. C'mon USPS & Royal Mail!